The present invention relates generally to an improved drum structure for use in copper foil production by an electrodeposition method. More particularly, the present invention relates to a drum structure which uses a titanium, zirconium or tantalum outer cylinder supported on an inner support cylinder wherein problems of differential movement or slippage and of electrical resistance between the two cylinders are minimized. Energy savings and higher rates of production are realized.
Electrodeposition of copper foil is one example of the general field of electroforming which is usually defined as the production of an article of electrodeposition upon a mandrel or form from which the electrodeposit is subsequently removed. Electroforming differs from ordinary electrodeposition or electroplating coatings in that electroforms are used as separate structures, rather than as coatings to provide decorative effects or corrosion resistence or the like.
In ordinary electroplating, such as the plating of tableware or the chromium plating of automobile bumpers, and the like, good adherence of the electrodeposited coating to the substrate metal is very important. For purposes of electroforming, including the production of copper foil, it is necessary that the electrodeposited metal temporarily be adherent to the substrate--during the course of deposition only--and that the adherence be sufficiently low to allow the electrodeposited foil to be readily removed without tearing at the conclusion of the deposition process.
As is well-known, the stripping surface is an important element in the formation of copper foil by electrodeposition methods. A number of metallic stripping surfaces and procedures for processing them have been proposed over the years for promoting the easy separation or stripping of electrodeposited metals from the substrate mold or mandrel. Metals that have been used include lead, silver, aluminum, chromium, copper, stainless steel, titanium and the like. Procedures such as so-called passivation of the surfaces with chromic solutions, anodic oxidation, oiling, iodizing, graphite application and the like have been used to enhance the ease of stripping.
Thin sheet copper or copper foil has been produced by electrodeposition on appropriate surfaces, on a commercial basis for almost fifty years. Its production and usage have become increasingly important with the rapid rise in the use of printed circuits in which electrodeposited copper foil is used in large quantities.
In the electrolytic production of copper foil, a horizontal drum or cylinder is partially immersed and rotated in a copper plating solution in which the drum is made cathodic. The speed of rotation and the cathode current density are coordinated so that the desired thickness of copper foil is deposited in the time the drum is immersed in the solution during its rotation. The emerging drum surface, now clad with electrodeposited copper foil, is washed and dried and the copper foil stripped from the surface and wound up in continuous lengths on an auxiliary roll.
The continuous production of electrodeposited copper foil and the continuous rotation of drums required therefor, makes copper foil production one of the more difficult examples of electroforming, especially with regard to the maintenance of the surface stripping layer and the continued reliability of said stripper layer in its function as a substrate for the electrodeposited metal to be repetitively stripped therefrom. As a result of these difficulties, only a few metals have been successfully used as drum working or stripping surface layers in the commercial production of electrodeposited copper foil.
Lead surface sheets were used for a number of years. Lead sheets 11/4 inches thick by 64 inches wide and 22 feet long, for example, were wrapped around lead wheels cast onto a copper sleeved shaft. The lead sheet was seamed and then fusion bonded to the wheels. Although this made a strong structural tie and also good electrical contact, for adequate stripping purposes it was found necessary to continuously polish the lead surface, in effect exposing a freshly polished lead surface for each rotation into the solution and electrodeposition of foil. This resulted in the production of a large amount of toxic lead powder. In addition, lead particles sometimes broke away from the surface and were incorporated into the surface of the copper foil from the drum. As soon as better surfaces were developed, lead drums were generally abandoned.
Stainless steel was proposed as a surface layer for copper foil production as long ago as the 1930's. However, it was not until about 1958 that large stainless steel drums were successfully used for commercial production of electrodeposition.
Stainless steel has advantages as a stripping surface, as follows:
1. It usually does not require continuous polishing and does not break away to be incorporated into the copper foil; and PA1 2. It can be joined to itself to make an excellent seam and, by brazing or welding, to steel or copper wheels for forming good electrical and mechanical bonding thereto.
However, stainless steel has the disadvantage that it is severely corroded when subject to electrochemical anodic action which occurs as a result of back EMF developed when power failure or other interruption in the applied direct current takes place. Stainless steel is presently used commercially as a drum surface layer, either by itself or with a chromium plated top surface, because of some advantages over previous surfaces.
Titanium was also proposed as a stripping surface for electroforming and is the subject of U.S. Pat. No. 2,646,391 issued in the name of Reginald S. Dean. Although titanium has been commercially used for stripping surfaces in such applications as starter cathode sheets in electrolytic zinc, maganese and copper refining, its high cost and poor electrical conductivity and especially the difficulty of making non-brittle welds between titanium and other metals, are disadvantages in the use of titanium for forming electrodeposition drums for use in copper foil production.
Early commercial attempts (1956) to avoid disadvantages of titanium with respect to its ability to be welded to other materials included wrapping titanium into a cylinder around a lead supporter drum. The resulting drum initially produced copper foil of good quality, except at the seams, as the welding thereat was defective. However, after some weeks of operation, difficulties were experienced: several hot spots developed and the titanium surface sheet showed evidence of separation from the support drum.
Another procedure was employed more recently by a Japanese Company in which a titanium cylinder was heat shrunk on to a mild carbon steel support structure. A number of these drums were put into production in Japan. Mild carbon steel was chosen for its strength coupled with acceptable electrical conductivity properties. The technique used to form the drum was basically to prepare a titanium cylinder of such dimensions that when heated and superimposed upon the inner steel drum, it would shrink and develop a substantial gripping force to hold the titanium cylinder on the steel drum. In some cases forces of about 540 tons are developed across the drum cylinder interface Steel was used for the support drum to insure that it would withstand forces of this magnitude. Because mild steel cannot withstand attack by copper sulphate-sulphuric acid electrolyte normally used in copper foil production, it was necessary to cover the ends of the steel drum as well as the outer surfaces.
Although slippage was minimized it was found that, in spite of the large forces created by the shrink fit mode of construction, the titanium layer may move with respect to the drum, after a period of use. When this occurs, it frequently results in the tearing of the protective sides and the leakage of electrolyte through the outer enclosure to attack the steel drum. In addition, because of the movement between the surfaces, "hot spots" were experienced occasionally on the titanium surface. These are not attributed to the separation of titanium from contact with the steel support drum, leading to higher currents and heating in adjacent areas which were still in contact. Such drums are therefore limited with respect to the current which can be used and accordingly to the rate of production of copper foil.
As is readily realized, the effect of separation of the outer cylinder from the support drum becomes greater as the amount of current flowing through the drum increases. Thus, these drums not only have the disadvantage of eventual physical failure and attack by electrolyte, but are limited by the amount of current that they carry and hence to the rate of production of copper foil.
It is a general object of the present invention to provide an improved drum construction for the electrodeposition of copper.
It is a more specific object of the present invention to provide a titanium, zirconium or tantalum covered drum which avoids the disadvantages of prior art drums.